Again: A ledger that is not decentralized is a bank database, not a cryptocurrency. As near as I can tell, Zuckbucks are nothing more than the JPMorganCryptocurrency but with a bigger consortium. The only difference seems to be who is given write privileges to the database.
The "decentralization" quality should not be used to describe any system that doesn't exhibit a permanent, irreversible systemic trend towards greater decentralization of all the levers, concentrations and bottlenecks of power within itself over time.
For this to happen, the natural tendency toward concentration of leverage would need to introduce a proportional net cost increase to the system, rather than (as it normally would) be the mechanism by which economies of scale accrue to it.